In a typical push-pull cable drive unit for use in a sunroof system, a drive pinion meshes with the push-pull cable, and a pair of cable guides are integrally provided in the casing for a speed reduction unit so as to support the cable and maintain the meshing of the pinion with the cable. For instance, refer to Japanese UM laid-open publication No. 58-192118.
Conventionally, the casing for a speed reduction unit was made of metallic material to achieve a desired durability. Such a metallic casing provides a high mechanical strength, but tends to be heavy and difficult to work on. To reduce weight and simplify the process of working the casing to a desired shape, it is desirable to form the casing of a speed reduction unit with plastic material.
In a sunroof system using a push-pull cable drive unit for converting a rotary motion into a linear motion, as shown in FIG. 4, a pair of runs of a push-pull cable 6 mesh with a pinion 5 which is connected to an electric motor via a speed reduction unit, and the rotation of the pinion 5 causes the two sections of the push-pull cable 6 to be driven longitudinally in opposite directions. The resulting longitudinal motion of the push-pull cable 6 is transmitted to the sunroof to effect the desired movement of the sunroof although it is not shown in the drawings. The force which the gear teeth of a pinion 5 applies to the cable 6 includes a longitudinal component which is effective in driving the push-pull cable 6. The force also includes a lateral component which urges the cable 6 away from the gear teeth as indicated by FX in FIG. 4. This component is not directly useful for driving the cable 6, and tends to push the two runs of the push-pull cable 6 away from each other. This lateral component is maximized when the cable 6 reaches a mechanical end of its stroke.
To prevent the two runs of the cable 6 from being excessive spread apart and disengaged from the pinion 5, a pair of cable guides 9 are integrally formed as a single piece construction with the casing of the speed reduction unit so as to oppose such a tendency. To achieve this goal, it is essential that the cable guides 9 are provided with an adequate rigidity. Cable guides 9 made of plastic material may not be able to provide an adequate rigidity. It is possible to increase the rigidity by providing ribs to the cable guides, but plastic material tends to be affected by temperature, and may not be able to provide a required rigidity under all conditions.